An IMS occupies a pivot position in the service processing for a communication network, and the high reliability of the IMS is the basis of the high reliability of the whole communication network. A serving call session control function (S-CSCF) serving as a network element (NE) for controlling the user service processing in the IMS acts as a core of the whole network service processing, so that a disaster tolerance capability of the S-CSCF is quite important to the high reliability of the IMS.
FIG. 1 is a flow chart that a user who intends to get access to an IMS network subscribes and registers in the IMS network in the conventional art.
In FIG. 1, once the user initiates a registration (1. REGISTER) at a terminal, a register request message is sent to a proxy CSCF (P-CSCF) assigned to the terminal by an access domain so as to enter an IMS subsystem. The P-CSCF forwards the register request to an interrogating CSCF (I-CSCF) of user's home domain according to a domain identity of the user (2. REGISTER).
The I-CSCF interrogates from a home subscriber serve (HSS) of the user home domain (3. UAR˜4. UAA), and a UAR message carries attribute value pairs (AVPs) as follows: Public-Identity, i.e., IMPU, User-Name, i.e., IMPI, Visited-Network-Identifier, i.e., information acquired from the REGISTER message, and User-Authorization-Type, i.e., a user authorization type, which is valued as REGISTRATION so as to represent an initial registration or re-registration. During the interrogation of this time, the I-CSCF acquires a “capability set of the S-CSCF for providing a service for a user” from the HSS, and the capability set is a group of numbers, which identifies the service processing capability requirements on the S-CSCF. The I-CSCF assigns an S-CSCF capable of supporting the required relevant service processing to the user according to the capability set requirements, and forwards the register request to the assigned S-CSCF (5. REGISTER).
Since it is the initial registration of the user, the S-CSCF needs to perform an authentication on the user. Firstly, the S-CSCF acquires data required by the authentication from the HSS (6. MAR˜7. MAA), and then delivers an authentication challenge to the terminal (8. 401˜10. 401), and the terminal calculates an authentication response and then re-initiates the registration (11. REGISTER˜12. REGISTER), and the register request carries the authentication response calculated by the terminal and a duration of a registration cycle expected by the terminal.
The I-CSCF interrogates from the HSS again (13. UAR˜14. UAA), and through the interrogation of this time, it acquires an address name of the S-CSCF currently performing the authentication on the user. The HSS records the address of the S-CSCF of the user when the S-CSCF requests the data required by the authentication of the user from the HSS (6. MAR˜7. MAA). After acquiring the address name of the S-CSCF currently performing the authentication on the user, the I-CSCF forwards the register request carrying the authentication response to the S-CSCF (15. REGISTER). After receiving the register request carrying the authentication response, the S-CSCF performs a verification on the authentication response, and if the authentication response is successfully verified, the S-CSCF formally records on the HSS that the user registration is successful, and requests to download subscription data of the user (16. SAR˜17. SAA). The S-CSCF returns a response that the register is successful to the user terminal (18. 200 OK˜20. 200 OK). In the 200 OK response, the S-CSCF provides a routing address of the S-CSCF itself in a Service-Route header field, and instructs the terminal to generate a Route header field of a session setup request message by utilizing the routing address in the Service-Route when a service request is initiated so as to ensure that the service request can be routed to the registered S-CSCF for being processed. In addition, the 200 OK response further provides the duration of the registration cycle determined through a final negotiation.
After the user registration is successful, the situations of recording the user registration data by each IMS NE are listed as follows. The HSS records the address of the S-CSCF with which the user registers. The S-CSCF records the subscription service data of the user, the duration of the registration cycle, the address of the P-CSCF where the user passes through when getting access to the IMS network, and a contact address of the user terminal. The P-CSCF records the address (i.e., Service-Route) of the S-CSCF for providing the service for the user, and the duration of the registration cycle. The I-CSCF does not record any user information.
FIG. 2 is a flow chart of setting up a session between a calling party and a called party registered in the IMS in the conventional art.
When initiating a session, the calling party determines the Route header field in a session setup request according to the Service-Route header field acquired during registration. After the session setup request is sent to the P-CSCF by the calling party, the P-CSCF routes, according to the Route header field (1. INVITE˜2. INVITE), the request to the S-CSCF with which the calling party registers. The S-CSCF routes the request message to the I-CSCF of a home domain of the called user according to a domain identity of the called user (3. INVITE). The called I-CSCF interrogates from the HSS to acquire, according to an identity of the called user (4. LIR˜5. LIA), the address of the S-CSCF with which the called user registers, and then forwards the session setup request to the S-CSCF (6. INVITE). After receiving the session setup request, the called S-CSCF updates a target address of the session setup request INVITE message by utilizing a contact address of the called user according to saved local registration data of the called user, and forwards the session setup request to the P-CSCF where the called user passes through when getting access to the IMS network (7. INVITE). After receiving the session setup request, the called P-CSCF forwards the session setup request to the called user (8. INVITE), and thus, one initial setup process of the end-to-end session in the IMS network is finished.
If the S-CSCF for providing the service for the user fails, the conventional art adopts a mechanism of using a registration timer to trigger a re-registration so as to assign a new S-CSCF to the user. FIG. 3 is a flow chart of triggering re-registration by a registration timer in the conventional art.
After registering with the IMS network successfully, the terminal immediately starts the registration timer according to the registration cycle determined through negotiation, and the registration timer triggers the re-registration of the user terminal when the timeout occurs. After the terminal initiates the re-registration, the I-CSCF and the P-CSCF route, according to the records of the initial registration of the terminal (1. Re-REGISTER˜5. Re-REGISTER), the register request to the S-CSCF1 with which the terminal registers. If the registered S-CSCF1 fails, the I-CSCF cannot receive any response, and as a result, a timeout occurs to a session initiation protocol (SIP) transaction. Since it is a re-registration process, the I-CSCF returns 408 Request Timeout to the terminal (6. 408˜7. 408), and after receiving the response, the terminal immediately initiates an initial registration (8. Ini-REGISTER˜9. Ini-REGISTER). The user has ever registered in the IMS network, so that the register request is still routed to the registered but currently failed S-CSCF1, and similarly, the I-CSCF cannot receive any response, and as a result, the timeout occurs to the SIP transaction. Since it is the initial registration, the I-CSCF reselects a new S-CSCF for the user, but firstly it is necessary to acquire the S-CSCF capability set requirements corresponding to the user subscription information from the HSS (13. UAR˜14. UAA). Then, the I-CSCF reselects an S-CSCF according to the capability set requirements, and forwards the register request to the new S-CSCF (15. REGISTER), thereby the process of reselecting an S-CSCF for the user affected by the failure is finished. Subsequently, the user finishes registering in the newly assigned S-CSCF according to the initial registration flow. After the user is successfully registered in the new S-CSCF, the IMS network service of the user restores to a normal status.
As known from the above descriptions about the mechanism of using the registration timer to trigger the re-registration, if the S-CSCF for providing the service for the user fails, the network service of the user cannot be restored until the registration timer of the user triggers the re-registration and the S-CSCF is reselected. That is to say, the service interruption duration of the user depends on the duration of the registration cycle of the user. The longer the registration cycle, the longer is the service interruption duration of the user. In order to meet the reliability requirements of the telecommunication network, the registration cycle is as short as possible. Unfortunately, if the duration of the registration cycle is set to be too short, it may result in frequent re-registrations. As for a network side, the frequent re-registrations may increase the processing burden of the network. Especially for the wireless access network, the frequent re-registrations excessively occupy the precious air interface resources. As for a user side, the frequent re-registrations cost the limited energy of a wireless terminal, and thereby shorten a standby time of the terminal.